This application is based on Japanese patent application HEI 10-45509 filed on Feb. 26, 1998, the whole contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a position detecting apparatus which obliquely observes alignment marks of a wafer and a mask and to a wafer/mask deformation error detecting method, and more particularly to a position detecting apparatus and a deformation error detecting method suitable for improving throughput of proximity exposure.
b) Description of the Related Art
A vertical detection method and an oblique detection method are known as a method of detecting the positions of marks on a wafer and a mask by using an aligner having a lens system combined with an image processing system. The vertical detection method observes position detecting marks along a direction perpendicular to the mask surface, and the oblique detection method observes it obliquely.
A chromatic bifocal method is known as a focussing method used by the vertical detection method. The chromatic bifocal method observes a wafer mark formed on a wafer and a mask mark formed on a mask by using light of different wavelengths and chromatic aberrations of the lens system, and focuses the images of the marks on the same flat plane. A wafer mark and a mask mark are hereinafter collectively called an alignment mark. An absolute precision of position detection by the chromatic bifocal method can be made high because the optical resolution of the lens system can be set high in principle.
However, since an alignment mark is observed vertically, a part of the optical system enters the exposure area. Since the optical system shields exposure light, it is necessary to retract the optical system from the exposure area when exposure light is applied. A time required for retracting the optical system lowers throughput. The alignment mark cannot be observed during the exposure, which is one of the reasons of lowering an alignment precision during the exposure.
With the oblique detection method, the optical axis of the optical system is disposed obliquely to the mask surface, and the system can be disposed without shielding the exposure system. It is therefore unnecessary to extract the optical system during the exposure, permitting observation of an alignment mark even during the exposure. Therefore, throughput does not lower and position misalignment during the exposure can be prevented.
A conventional oblique detection method uses oblique focussing in which regular reflection light reflected from the wafer and mask marks is obliquely focussed to detect the images of the marks. An absolute precision of position detection is therefore lowered by image distortion. Furthermore, since regular reflection light is incident upon an observation lens, the optical axis of illumination light cannot coincide with the optical axis of observation light. Since the optical axes of illumination and observation light are required to be separated, if there is even a slight shift between both the axes, the image is distorted and the detection precision is lowered.
The oblique detection method obliquely observes and focuses wafer and mask marks, the absolute precision of position detection is lowered by image distortion. Further, since the optical axes of illumination light and observation light are not coincide, theses axes cannot be disposed coaxially. Therefore, the illumination optical axis is likely to shift from an ideal optical axis. If the illumination optical axis shifts from the ideal optical axis, the image is distorted and correct position detection becomes difficult.
It is an object of the present invention to provide a position detecting apparatus and method capable of high precision position alignment even during exposure without lowering throughput.
According to one aspect of the present invention, there is provided a position detecting apparatus comprising: holding means for holding a wafer with an exposure surface having first and second wafer marks formed thereon for scattering incidence light for position alignment and a mask with a mask surface having first and second mask marks formed thereon for scattering incidence light for position alignment, the wafer and the mask being faced each other with a predetermined distance being set between the exposure surface and the mask surface, the first and second mask marks being in correspondence with the first and second wafer marks, respectively, and the first and second mask marks and the first and second wafer marks constituting first and second alignment mark groups; first and second illumination optical systems for applying illumination light to the first and second alignment mark groups of the wafer and the mask held by the holding means, along an optical axis which is oblique relative to the exposure surface; first and second observation optical systems having light reception surfaces on which scattered light from the first and second alignment mark groups is focussed, optical axes of the first and second observation optical systems being oblique relative to the exposure surface of the wafer, and the optical axes vertically projected upon the exposure surface both crossing at a right angle a first virtual straight line interconnecting the first and second wafer marks; and control means for controlling to detect a difference of a size between the wafer and the mask in a direction of the first virtual straight line, in accordance with images obtained by the first and second observation optical systems and formed by scattered light from the first and second alignment mark groups.
A relative position of the first wafer and mask marks in the direction of the first virtual straight line can be detected from images obtained by the first observation optical system. In accordance with the detected relative position information, position alignment of the wafer and mask in the first virtual straight line direction can be performed. A relative position of the second wafer and mask marks in the direction of the first virtual straight line can be detected from images obtained by the second observation optical system. In accordance with the detected two relative positions measured by the first and second observation optical systems, a difference of the deformation amount, especially a magnification error, between the wafer and mask in the first virtual straight line direction can be obtained.
According to another aspect of the present invention, there is provided a position detecting method comprising: a holding step of holding a wafer with an exposure surface having first and second wafer marks formed thereon for scattering incidence light for position alignment and a mask with a mask surface having first and second mask marks formed thereon for scattering incidence light for position alignment, the wafer and the mask being faced each other with a predetermined distance being set between the exposure surface and the mask surface, the first and second mask marks being in correspondence with the first and second wafer marks, respectively, and the first and second mask marks and the first and second wafer marks constituting first and second alignment mark groups; an illuminating step of applying illumination light to the first and second alignment mark groups of the wafer and the mask held at the holding step, along an optical axis which is oblique relative to the exposure surface; a focussing step of focussing scattered light from the first and second alignment mark groups on first and second light reception surfaces of first and second observation optical systems by using the first and second observation optic al systems, optical axes of the first and second observation optical systems being oblique relative to the exposure surface of the wafer, and the optical axes vertically projected upon the exposure surface both crossing at a right angle a first virtual straight line interconnecting the first and second wafer marks; and a detecting step of detecting a deformation difference between the wafer and the mask in a direction of the first virtual straight line, in accordance with images focussed by the focussing step and formed by scattered light from the first and second alignment mark groups.
As described above, positions can be detected at a high precision through oblique observation of scattered light from the wafer and mask marks. Since the optical systems are not required to be disposed in the exposure area, the exposure can be performed without retracting the optical systems from the exposure area. It is possible to improve throughput. It is also possible to detect the position shift of patterns to be caused by deformation of the wafer and mask.